1. Field of the Invention
The present invention is generally related to the area of optical communications. More particularly, the present invention is related to optical devices with fiber Bragg grating features but structured with thin film filters and the method for making the same.
2. The Background of Related Art
A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
It is well known that using a photo sensitive fiber core material and a strong UV irradiation in a patterned mask, periodic refractive index change induced by the mask generated interference pattern along the fiber core can create a FBG which retro-reflects a band of spectrum from an incoming signal. FIG. 1 shows an example of a section of fiber with a Bragg reflector. FBG has been used to create the wavelength division multiplexing (WDM) system where multiple such FBG's are used, each of which rejects or demultiplexes a narrow band of WDM signals. However, due to the fact that (1) FBG's fabrication is slow and non-mass production oriented; and (2) FBG must be used in conjunction with a fiber circulator to separate the demultiplexed signals from the input signal stream and the circulator is very expensive, the FBG's became less popular to other WDM technologies, such as those based on thin film filters (TFF).
TFF technology has been popular and gaining momentum in recent years. However, TFF exhibits some properties in almost reverse fashion as FBG does. While FBG rejects a narrow band but passes the remaining spectrum, TFF does oppositely by passing a narrow spectrum and reflects the remaining spectrum portions. For certain applications where rejection of a narrow spectrum in a retro-reflection fashion is preferred, markets expect something that can preserve this property but to be as cost effective as TFF can support.
FIG. 2 shows another prior art known as a TFF based WDM 3-port filter. A TFF is made by dielectric coatings on one side of a glass substrate in many layers of alternating high and low indices. To make a final TFF device for WDM applications, this filter is sandwiched between a pair of collimating lenses so that the light impinging upon the filter will be sufficiently collimated or close to plane wave approximations. Fibers marked as Input (I), Reflection (R) and Transmission (T) ports, are introduced by fiber pigtails formed as glass capillaries hosting fibers to the device as input and output ports. Here the TFF is shown to be attached to the lens which could be either a gradient index (GRIN) lens that has a flat end surface as shown in the main part of FIG. 2 or it is fixed to a glass sleeve over a lens of a curved end-face as shown in the inlet of FIG. 2. Either way, this prior art shows the device has 3 ports instead of 2 ports shown in the FBG filter case. Operationally, a narrow band spectrum signal passes from left through I-port to the right side or to T-port. The remaining spectrum will be reflected by the TFF to the R-port. To help visualize the spectrum, a typical spectrum for input (I), Transmission (T) and reflection (R) ports, respectively, are respectively shown in FIG. 3. As far as that for FBG is concerned, one can expect a similar spectrum, but within 2 ports instead of three. The spectrum of I and T in FIG. 3 will be all in the input fiber but in the two opposite directions of the light propagation. What appears to be at the right hand side fiber port will be the spectrum of R in the TFF configuration shown in FIG. 2.
FIG. 4 depicts FIG. 1 and FIG. 4 of U.S. Pat. No. 6,400,862 in which a traditional 3-port TFF based WDM device is reconfigured to fold a Transmission port (e.g., the right side of the device shown in FIG. 2) to the left side of the device. The reason for making such a folding is due to the fact that all fibers would have a bend induced light loss and also due to the concern of incurring excess loss while single mode fibers are designed to have a minimum bend radius >R=25 mm. This restricts the integration dimension that a 3-port filter offers because fibers on both sides will need to observe this bend radius. Reserving 25 mm extra length that cannot be used for other functional purposes sometimes is prohibitive.
As shown in FIG. 4, one can see that instead of letting the fiber of the T-port go straightforward to the right side of the device, the light beam passing the TFF filter is folded back by a mirror so that it passes the TFF again and then through the same lens and be aligned to go into the third fiber at the same fiber pigtail device. This device can be integrated in smaller form factors as the integrator only needs to route fibers from one side of the device inside a module.
Accordingly, there is a great need for techniques for optical devices in the structure of a fiber Bragg grating device, but at the same time, the devices so designed are amenable to small footprint, enhanced impact performance, lower cost, and easier manufacturing process.